(1) Field of the Invention
This invention relates to printing ink used for lithographic (planographic) printing as well as offset printing. More particularly, the invention relates to a vehicle of lithographic printing ink, a binder resin used for preparing the ink vehicle and a method for producing the binder resin. In the present invention, the molecular weight of the binder resin is made so high that the misting is reduced and the workability in polymerization process for the binder resin is improved by maintaining the melt viscosity of the binder resin below a certain level.
(2) Prior Art
The lithographic printing inks in the prior art, especially those used for sheet-fed printing and for heat-set printing, are generally prepared by adding suitable pigments to a composition of a binder resin typically exemplified by rosin-modified phenol resin, petroleum resin or alkyd resin; drying oil or semi-drying oil such as linseed oil, tung oil or safflower oil; and a vehicle mainly comprising a high boiling petroleum solvent, and they are kneaded using a mixing apparatus such as a three-roll ink mill. Among the above-mentioned petroleum solvents, the aromatic petroleum solvents are hitherto employed widely because they are excellent in solvent power to the binder resin, in addition, they are inexpensive.
In recent years, as the environmental protection is attracting considerable attention in the industry, it is investigated to replace the aromatic solvents formerly used for preparing printing inks with non-aromatic solvents, especially, naphthenic solvents which are desirable in view of the problems in environmental protection in printing job sites. The non-aromatic solvents are, however, inferior to the conventionally used aromatic solvents in the solvent power to binder resins, so that it is difficult to impart the gloss to printed images by replacing only the aromatic solvents with non-aromatic solvents. As a countermeasure in this regard, it has been proposed to form oxidation polymerization layers on printed materials by increasing the content of drying oil in a printing ink. However, in the printing ink of this kind, some other problems are pointed out such as the lowering in drying property and heat-setting property.
For Example, in European Patent No. 697 446, it is proposed that modified petroleum resins having weight average molecular weights of 30,000 to 200,000 are prepared as binder resins by reacting the maleic modified resin of the copolymer of dicyclopentadiene (DCPD) and .alpha.-olefin with phenol resin prepared from butyl phenol, and they are used for the vehicles of lithographic printing inks containing non-aromatic hydrocarbon resins.
In recent years, with the tendency to use high speed printing machines, the problem of misting became conspicuous. For example, in an offset rotary press, the problem of misting is serious because even a rotational speed as high as more than 1000 rpm is often adopted.
There is a tendency that the misting is reduced by using a resin of higher molecular weight. However, when the molecular weight of polymer is simply increased, it is apprehended that the melt viscosity of resin is raised, as a result, the workability in bulk polymerization is impaired. For example, in the case of the binder resin as disclosed in the foregoing European Patent No. 697 446, when its molecular weight is simply increased, the melt viscosity is also raised with the rise of the molecular weight, so that such a resin cannot be used because the workability in polymerization is impaired.